1. Field of the Invention
The present invention relates to an optical information recording medium capable of recording and reproducing information at high speed and high density by an optical means, such as laser, more particularly a barrier layer for preventing deterioration of the medium as a result of the reactions of the reflecting layer with the dielectric protective layer and a chalcogen-based component in the recording layer, when a highly thermoconductive material of Ag or with Ag as the major ingredient is used for the reflecting layer for high-speed recording, and applicable to and developable in the fields which need weather resistance of molded metallic shapes or metallic films.
2. Description of the Related Art
Of the recording-type optical disks, the one depending on phase change in general is basically and functionally structured by 4 layers, for example, plastic substrate/dielectric material/chalcogen-based, phase-change recording material/dielectric material/cooling/reflecting layer of Al- or Ag-based alloy, where the chalcogen-based phase-change recording material takes the crystalline or amorphous structure depending on temperature to which it is exposed, and information recorded can be recognized by the resulting difference in reflectivity.
Recently, the recording-type optical disks depending on phase-change have been frequently used, which increasingly needs higher density for recording and higher linear velocity of writing.
Various proposals have been made for increasing recording density. One example of these proposals uses semiconductor laser beams of short wavelength and large numerical aperture (NA) of the optical pick-up to narrow down the recording laser beams for high-density recording (International Publication No. WO99/00794, Claims 9 to 11 in page 2, line 21 to page 3, line 3, and page 10, line 9 to line 12).
For increasing linear writing velocity, the reflecting layer plays a vital role. It is required to keep high reflectivity and rapidly cool down, and is in many cases made of a material of high thermoconductivity, e.g., Ag, Au or Cu, in particular Ag.
However, Ag as a reflecting layer material involves problems because of its chemical sensitivity to non-metallic elements, e.g., chlorine and oxygen, and their ions, resulting in contamination therewith and insufficient weather resistance in the environment or atmosphere in which the recording medium is used.
One of the proposed techniques to solve the above problems is doping Ag with a metallic element as an impurity. More specifically, it is doped with 0.1% to 5.0% of Au as an impurity and another specific metallic element also as an impurity (Japanese Patent Application Laid-Open (JP-A) No. 2002-129260, Claims, page 2, column 1, line 1 to line 24).
When at least an Ag-based material is used as a constituent material for the reflecting layer, on the other hand, there are basic problems arising in relation to the other layers.
The important considerations for the dielectric layer materials are thermal, optical and productivity-related properties (e.g., film-forming rate), and ZnS/SiO2 (80/20 by mol) has been extensively used because it satisfies these requirements.
However, it is known that a metal of high reflectivity and thermoconductivity, such as Ag, Au or Ag-based one, for the cooling/reflecting layer causes characteristic problems of corrosion by sulfur, when the dielectric layer contains sulfur. These problems are particularly noted with an Ag-based material.
One of the proposals for preventing corrosion of the reflecting layer of Ag is placing an intermediate layer between the dielectric layer containing sulfur atom and reflecting layer containing Ag as the major ingredient, where the intermediate layer is formed of oxide, nitride or carbide of a metal or semiconductor, or amorphous carbon. (Japanese Patent Application Laid-Open (JP-A) No. 11-238253, Claim 1, page 2, column 1, line 2 to line 11, and paragraph [0046], page 7, column 12, line 45 to page 8, column 13, line 1).
Another proposal for preventing sulfidation of the reflecting layer material is placing a barrier layer between the dielectric and reflecting layers in a manner similar to that proposed by the above-mentioned patent document JP-A No. 11-238253, where the barrier layer is formed of nitride, oxide, carbide or oxynitride of an element α (α is at least one element selected from Sn, In, Zr, Si, Cr, Al, Ta, V, Nb, Mo, W, Ti, Mg and Ge) (Japanese Patent Application Laid-Open (JP-A) No. 2002-74746, Claim 1, page 2, column 1, line 2 to line 11, and Claim 6, page 2, column 1, line 23 to line 25).
However, the intermediate layer proposed by JP-A Nos. 11-238253, and 2002-74746 involves its own problems.
The barrier layer of oxide involves productivity-related problems, because of its lower film-forming rate than that associated with the dielectric layer of ZnS/SiO2 where ZnS is 80% by mol and SiO2 is 20% by mol, which has been extensively used.
The inventors of the present invention have confirmed, based on the tests carried out by themselves, that it is formed at a film-forming rate of 1/15} to 1/3 that of the dielectric layer of ZnS/SiO2 formed using the same apparatus by RF sputtering, and 3/5 to 4/5 that of the reflecting layer material formed by DC sputtering, indicating that it is much lower in productivity than that of ZnS/SiO2.
In the case of the barrier layer of nitride, its film-forming rate is 1/12 to 4/5 that of ZnS/SiO2 layer, the upper limit being slightly higher than that of the oxide-based layer.
However, particularly noted is its optical constant k value, roughly by 1 to 4 digits higher than that of the oxide-based layer. This, in turn, conceivably causes the problems of decreased reflectivity and sensitivity for the recording medium signals, requiring higher writing power.
The barrier layer of carbide-based material has an optical constant k value higher than that of the oxide-based one, as is the case with the nitride-based one, with the result that its reflectivity for the recording medium signals decreases, deteriorating its sensitivity.
Moreover, a carbide-based material is extensively used for molds for pressed glass lenses or mold surface layers. Therefore, it is conceivably insufficient in adhesion to a chalcogenide-based material for the recording layer or to a dielectric layer material similar to glass material in many cases.
Moreover, the carbide-based layer, although high in resistance to thermal shock, is generally high in thermal conductivity to dissipate energy power from the semiconductor laser to the reflecting layer via the carbide layer, which conceivably increases required writing power. This limits thickness of the film.
When laser beams of short wavelength (e.g., 405 nm) are used to increase recording density, optical constant n or k of the dielectric or barrier layer varies with the wavelength, generally tending to increase as the wavelength decreases.
For each of the optical information recording medium layers to have a similar optical thickness for high-density recording after it is actually formed, it is necessary to be sufficiently thin. However, in particular the barrier layer should physically have a certain thickness to prevent pinholes or mass diffusion therein, and may encounter problems of being difficult in exhibiting its functions when it is excessively thin.
Advantages and Objects
It is therefore an object of the present invention to provide an optical information recording medium good in disk characteristics to cope with demands for recording at high linear velocity and sensitivity, and, at the same time, excellent in can-stability.
It is another object of the present invention to provide an optical information recording medium exhibiting an effect of preventing sulfidation-caused corrosion of the reflecting layer, in particular that of an Ag-based material, and keeping production efficiency and also recording characteristics as an optical disk.